How this calculator works
A 12 V circuit has little voltage to spare: a 3% allowance is only 0.36 V. The calculation includes both supply and return conductors. Choose copper, aluminium or a measured resistivity in nano-ohm metres at 20 C. The comparison table keeps source power, one-way distance, temperature and percentage drop unchanged; doubling supply voltage halves current and quarters required area. It assumes equal resistance in both wires and excludes connectors and chassis returns. The AWG result is an area comparison; it does not check cable heating or fuse selection.
Formula
DeltaV = k rho(T) L I/A; A = k rho(T) L I/DeltaV; L = DeltaV A/(k rho(T) I); I = DeltaV A/(k rho(T) L). For two wires k = 2; for balanced three-phase k = sqrt(3). rho(T) = rho20[1 + alpha(T - 20 C)].
Worked example
A 10 A load over 10 m one-way at 12 V with a 3% limit needs 9.3333 mm² of copper at 20 C. The same 120 W source power at 24 V draws 5 A and needs 2.3333 mm² for the same run and percentage limit.
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Frequently asked questions
Should I enter the battery voltage while charging?
This page fixes the source at nominal 12 V. For a measured charging or low-battery voltage, use the general wire size calculator.
Is the distance the full cable loop?
Enter the distance from source to load. The model adds an equally long return conductor automatically.
Why might an LED strip still be dim?
Strip traces, terminals and connectors add resistance beyond the two conductors modelled here. Check the voltage at the strip under load.